Fair Value of Financial Instruments - Valuation of Financial Instruments by Pricing Observability Levels (Details) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities	$ 9,505,000	$ 50,365,000
Convertible securities	18,460,000	156,685,000
Fixed income securities	48,654,000	48,651,000
Taxable securities	111,591,000	312,753,000
Tax-exempt securities	416,966,000	559,704,000
Short-term securities	33,068,000	68,717,000
Mortgage-backed securities	121,794,000	125,065,000
U.S. government agency securities	188,140,000	244,046,000
U.S. government securities	7,729,000	2,549,000
Derivative contracts	35,027,000	47,826,000
Derivative contracts	(377,436,000)	(406,185,000)
Total financial instruments and other inventory positions owned	990,934,000	1,616,361,000
Equity securities	15,740,000	154,589,000
Fixed income securities	39,909,000	21,460,000
U.S. government agency securities	21,267,000	27,735,000
U.S. government securities	159,037,000	523,527,000
Derivative contracts	3,202,000	10,813,000
Derivative contracts	(391,446,000)	(428,033,000)
Total financial instruments and other inventory positions sold, but not yet purchased	239,155,000	738,124,000
Securities posted as collateral to its counterparties	0	0
Level I
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities	7,569,000	39,191,000
U.S. government securities	7,729,000	2,549,000
Derivative contracts	164,000	76,000
Total financial instruments and other inventory positions owned	15,462,000	41,816,000
Cash equivalents	130,138,000	1,562,000
Investments at fair value	34,874,000	20,704,000
Total assets	180,474,000	64,082,000
Equity securities	13,489,000	153,254,000
U.S. government securities	159,037,000	523,527,000
Derivative contracts	149,000	189,000
Total financial instruments and other inventory positions sold, but not yet purchased	172,675,000	676,970,000
Level II
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities	1,936,000	11,174,000
Convertible securities	18,460,000	156,685,000
Fixed income securities	48,654,000	48,651,000
Taxable securities	105,775,000	312,753,000
Tax-exempt securities	415,789,000	558,518,000
Short-term securities	32,348,000	67,997,000
Mortgage-backed securities	670,000	316,000
U.S. government agency securities	188,140,000	244,046,000
Derivative contracts	412,299,000	453,795,000
Total financial instruments and other inventory positions owned	1,224,071,000	1,853,935,000
Total assets	1,224,071,000	1,853,935,000
Equity securities	2,251,000	1,335,000
Fixed income securities	39,909,000	21,460,000
U.S. government agency securities	21,267,000	27,735,000
Derivative contracts	387,351,000	430,835,000
Total financial instruments and other inventory positions sold, but not yet purchased	450,778,000	481,365,000
Level III
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Taxable securities	5,816,000
Tax-exempt securities	1,177,000	1,186,000
Short-term securities	720,000	720,000
Mortgage-backed securities	121,124,000	124,749,000
Derivative contracts		140,000
Total financial instruments and other inventory positions owned	128,837,000	126,795,000
Investments at fair value	107,907,000	74,165,000
Total assets	236,744,000	200,960,000
Derivative contracts	7,148,000	7,822,000
Total financial instruments and other inventory positions sold, but not yet purchased	7,148,000	7,822,000
Measured on a recurring basis
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities	9,505,000	50,365,000
Convertible securities	18,460,000	156,685,000
Fixed income securities	48,654,000	48,651,000
Taxable securities	111,591,000	312,753,000
Tax-exempt securities	416,966,000	559,704,000
Short-term securities	33,068,000	68,717,000
Mortgage-backed securities	121,794,000	125,065,000
U.S. government agency securities	188,140,000	244,046,000
U.S. government securities	7,729,000
Derivative contracts	35,027,000	47,826,000
Total financial instruments and other inventory positions owned	990,934,000	1,616,361,000
Cash equivalents	130,138,000	1,562,000
Investments at fair value	142,781,000	94,869,000
Total assets	1,263,853,000	1,712,792,000
Equity securities	15,740,000	154,589,000
Fixed income securities	39,909,000	21,460,000
U.S. government agency securities	21,267,000	27,735,000
U.S. government securities	159,037,000	523,527,000
Derivative contracts	3,202,000	10,813,000
Total financial instruments and other inventory positions sold, but not yet purchased	$ 239,155,000	$ 738,124,000